Peter Tatchell, coordinator of the Equal Love campaign, said heterosexual couples faced "legal inequality and discrimination" because under the terms of the legislation published today they will not have the option of getting civil partnerships instead.

The marriage (same sex couples) bill enables gay couples to convert their civil partnerships into marriage if they wish. Ministers want the option to be phased out completely, but face opposition from one of the gay community's loudest voices.

"Despite proclaiming that the legalisation of same-sex civil marriage is driven by the principle of equality, the government's forthcoming legislation will retain the inequality of the current legal ban on heterosexual civil partnerships," Tatchell said.

"Some straight men and women don't like the patriarchal traditions of marriage. They'd prefer a civil partnership. Why shouldn't they have that option?"

His objections are unlikely to be heeded by ministers, who face much bigger challenges as they seek to get the legislation on the statue books by the summer.

The bill ensures religious organisations which are unwilling to conduct same-sex marriages are able to refuse gay couples without risking legal challenge in the courts.

But David Cameron and his coalition colleagues face a large-scale rebellion from Conservative MPs uncomfortable with the reform. Campaign groups like the Coalition For Marriage are placing intense pressure on backbenchers, having notched up over 600,000 signatures for their petition against same-sex marriage.

Defence secretary Philip Hammond has reportedly attempted to derail the bill by questioning the robustness of the legislation's opt-out for religious organisations.

Other Cabinet members including environment secretary Owen Paterson and Welsh secretary David Jones are also thought to be enemies of the reform.

Leader of the Commons Andrew Lansley announced yesterday the bill would be debated for the first time on Tuesday February 5th. The Cabinet split will be resolved by providing MPs with a free vote in the Commons.

"Our proposals recognise, respect and value the very important role that faith plays in our lives," equalities minister Maria Miller said.

"I have always been crystal clear that I would not put forward any legislation that did not provide protection for religious organisations.

"This bill protects and promotes religious freedom, so that all religious organisations can act according to their doctrines and beliefs."

The legislation provides a 'quadruple lock' to this effect. It will become illegal for any religious organisation or individual minister to be forced to marry same-sex couples. Instead both religious organisations and individual ministers will have to expressly opt-in.

The Church of England will be explicitly protected by making it illegal for the institution to marry same-sex couples.

And equality legislation will also be amended so no discriminatory claims can be brought.

Despite the coalition's determination to get the legislation passed quickly, enabling same-sex couples to marry as soon as this summer, the government may find it harder than it expects to get the bill through parliament.

While support from Labour means the reforms will pass the Commons easily, the Lords may pose a greater threat. Over half of members of parliament's upper House have said they believed the government's legislation should "not proceed", a poll by ComRes for the Coalition for Marriage group found.